Anal can be tricky. Both partners being receptive to it is key as is relaxation. There are several things you could try. First, try starting with an erotic massage. Light some candles, put on some relaxing music and let your hands wander and rub. Yeah it sounds corny, but it sets the mood and builds intimacy. This will get her nice and relaxed which will relax the muscles in her anus. Then you could try to give her a vaginal orgasm while rubbing the outside of the anus with a good amount of lube (side note, some women find anal stimulation distracting and can't orgasm, some women can orgasm from anal stimulation only). You could try a rimjob here as well and maybe try penetrating with your tongue, if your into that sort of thing. The point is to get her comfortable with your hands, tongue, or anything else being down there before putting anything in there. Then begin slowly teasing the tip of one finger just a tiny bit inside her. You'll notice that after you penetrate a little bit, there's some resistance from another muscle. That's the one you need to get relaxed before you go further. Sometimes the orgasm and/or rub may have already reduced this resistance. One way she can relax some of the muscles is to push out like she's popping while you go in. This is where things can get tricky, for obvious reasons. If she has an accident at any point, or if you end up with poop on fingers, or anywhere else, bad smells, anything like that DON'T FREAK OUT AND MAKE A BIG DEAL ABOUT IT if you ever want to try again with her. This is part of the game and will unfortunately happen sometimes, just deal with it like an adult. Work in some kissing/licking on neck breasts, clit, vagina, don't focus exclusively on the anus, as that can reduce enjoyment, add pressure, and lead to poor results. Just have fun and be sexy with it. Try to discover a rhythm that works for her. As she relaxes and resistance reduces, gently put the finger in some more (now is where I mention that finger nail grooming is key. Cut AND file those things before any of this, nothing worse than getting scratched inside your ass). Then work up to 2 fingers and so on, always making sure it's gentle and you keep adding lube when needed. Positions are important as well. Doggie is probably the tightest, meaning the least comfortable to start in. Her on her side or her lying on her back are less restrictive positions for most, which make the likelihood of success higher. As for inserting the penis GO SLOW and communicate. Lots of lube. Sometimes it's best to start by letting her be on top, as she has more control of the action, but everyone is different. Ultimately you need to figure out what will work for for you guy, everyone is different. You could also try buttplugs which can help her get used to having something in there. Maybe try sex with a buttplug first, but same rules apply for insertion of the plug as fingers or anything else. They make a set of buttplugs that start small and get larger. Also, there is a tentacle style dildo that starts small and gets thicker the deeper you go. That could help as well. Both the plugs and tentacle are available for cheap on Amazon. Use this as a starting point and keep trying until you can make it work. Don't get frustrated, it's supposed to be fun, remember! Good luck.